Electrode Polarity Reversal Research Articles

Engineering Electrode Polarity for Enhancing In Situ Generation of Hydroxyl Radicals Using Granular Activated Carbon.

Recently, granular activated carbon (GAC) has shown its effectiveness as a cathode material for in situ ROS generation. Here, we present an electrochemically modified GAC cathode using electrode polarity reversal (PR) approach for enhanced H2O2 decomposition via 2-electron oxygen reduction reaction (2e-ORR). The successful GAC modification using PR necessitates tuning of the operational parameters such as frequency, current, and time intervals between the PR cycles. This modification enhances the GAC hydrophilicity by increasing the density of surface oxygen functionalities. After optimization of the electrode polarity, using the 20 (No PR)-2 (PR) interval and 140 mA current intensity, the •OH concentration reaches 38.9 μM compared to the control (No PR) (28.14 μM). Subsequently, we evaluated the enhanced •OH generation for the removal of glyphosate, a persistent pesticide used as a model contaminant. The modified GAC using PR removed 67.6% of glyphosate compared to 40.6% by the unmodified GAC without PR, respectively. The findings from this study will advance the utilization of GAC for in situ ROS synthesis, which will have direct implications on increasing the effectiveness of electrochemical water treatment systems.

Removal of Organic Pollutants Via Electrochemical Oxidation Near Anodes During Electrokinetic Remediation: Proof of Concept

This study designed a practical approach of separating electrodes chambers to prove the influence of electrochemical oxidation on pyrene removal during electrokinetic (EK) remediation. The anode and cathode electrodes were arranged in two independent soil chambers, and the soil chambers were connected by salt bridges. Salt bridges electrokinetic (SBEK) remediation avoided acid–base neutralization and facilitated the intensity of the current. The results showed that 99.5% of pyrene was removed within 7 days in anodic chamber. The removal rate of pyrene was ∼99.6–99.9% at SBEK1 anode and 29.5–36.2% at SBEK1 cathode after 42 days. The changes in current, pH, and redox potential values indicated that an efficient “oxidizer” was formed in the anode chamber. Compared with conventional EK remediation, SBEK can take place under a suitable oxidizing condition because of increasing current intensity and decreasing soil pH in the anode chamber. Furthermore, SBEK with electrode polarity reversal test had better decontamination efficiency of pyrene than fixed electrodes. Removal rate of pyrene increased to 62.9% after 14 days with polarity reversal; correspondingly, the energy consumption of SBEK test was ∼13 times higher than that of the conventional EK test. The novel application of separated electrodes chambers appears to be a viable strategy for removing organic pollutants.

Radical–Radical Cross-Coupling Assisted N–S Bond Formation Using Alternating Current Protocol

Radical–Radical Cross-Coupling Assisted N–S Bond Formation Using Alternating Current Protocol

Mitigating Electrode Fouling in Electrocoagulation by Means of Polarity Reversal: The Effects of Electrode Type, Current Density, and Polarity Reversal Frequency

One of the biggest issues in electrocoagulation (EC) water treatment processes is electrode fouling, which can cause decreased coagulant production, increased ohmic resistance and energy consumption, and reduced contaminant removal efficiency, among other operational problems. While it has been suggested that switching the current direction intermittently (i.e., polarity reversal, PR) can help mitigate electrode fouling, conflicting results about the utility of this approach have been reported in the literature. The objective of this study was to systematically investigate the effects of PR frequency and current density on the performance of Fe-EC and Al-EC. It was found that operating Fe-EC under the PR mode reduced neither electrode fouling nor energy consumption. Notably, the Faradaic efficiency (ϕ) in Fe-EC decreased with increasing PR frequency; ϕ was as low as 10% when a PR frequency of 0.5 minutes was employed. Unlike Fe-EC, operating Al-EC under the PR mode resulted in high coagulant production efficiencies, reduced energy consumption, and diminished electrode fouling. In addition to comparing PR-EC and DC-EC, a novel strategy to minimize electrode fouling was investigated. This strategy involved operating Fe DC-EC and Al DC-EC with a Ti-IrO2 cathode, whose fouling by Ca- and Mg-containing minerals could be readily avoided by periodically switching the current direction.

High-Performance Ti/IrO2–RhOx–Ta2O5Electrodes for Polarity Reversal Applications

Electrode polarity reversal has been widely employed to inhibit membrane fouling and induce scale detachment during electrodialysis and electrochemical precipitation, respectively. However, frequent polarity reversal shortens the lifetime of traditional dimensionally stable anodes. In this study, a novel titanium electrode that employed a ternary iridium, tantalum, and rhodium oxide mixture (IrO₂–RhOx–Ta₂O₅) as an active electrocatalyst film was investigated for polarity reversal applications. It was found that the Ti/IrO₂–RhOx–Ta₂O₅ electrode with an Ir/Rh/Ta molar ratio of 3:3:4 had a service lifetime of 1030 h in a 20 g L–¹ Na₂SO₄ solution at a current density of 2000 A m–² and a polarity reversal frequency of 12 h–¹. In contrast, a Ti/IrO₂–Ta₂O₅ electrode with an Ir/Ta molar ratio of 6:4 under the same conditions exhibited a service lifetime of only 275 h. Physicochemical characterization revealed that the IrO₂–RhOx–Ta₂O₅ film has a compact microstructure, solid solution characteristics, and contains both Rh³⁺ and Rh⁴⁺ which contributes to the stability during polarity reversal. Moreover, the Ti/IrO₂–RhOx–Ta₂O₅ electrode showed better chlorine evolution performance than the Ti/IrO₂–Ta₂O₅ electrode.

Arsenic detoxification by iron-manganese nodules under electrochemically controlled redox: Mechanism and application

Iron-manganese binary oxides are characterized by high oxidation and adsorption capability and widely applied to arsenic (As) detoxification in contaminated waters. Despite of their lower preparation cost relative to synthesized iron-manganese binary oxides, the low adsorption capacity of natural iron-manganese oxides largely hinders their application. Here, electrochemically controlled redox was employed to improve the As(III,V) removal performance of iron-manganese nodules in a symmetric electrode system, and the removal mechanism and electrode reusability were also examined. Experimental results showed that both the electrochemical reduction and oxidation of birnessite in iron-manganese nodules contributed much to As(III,V) removal. Higher cell voltage facilitated a higher removal efficiency of total As within 0–1.2 V, which reached 94.7% at 1.2 V for actual As-containing wastewater (4068 μg L−1). The efficiency was obviously higher than that at open circuit (81.4%). Under electrode polarity reversal, the alternating reduction dissolution and oxidation recrystallization of birnessite in iron-manganese nodules promoted their contact with As, enhancing the total As removal efficiency from 75.6% to 91.8% after five times of repeated adsorption. This research clarifies the effect of electrochemical redox on As(III,V) detoxification by iron-manganese oxides, and expands the application of natural iron-manganese nodules in the treatment of As-contaminated wastewaters.

Efficient H2O2 electrogeneration at graphite felt modified via electrode polarity reversal: Utilization for organic pollutants degradation

Electrochemical synthesis of H2O2 offers a great potential for water treatment. However, a significant challenge is the development of efficient cathode materials for the process. Herein, we implement a practical electrochemical cathode modification to support efficient H2O2 electrogeneration via the reduction of dissolved anodic O2. Graphite felt (GF) is in situ anodically modified by electrode polarity reversal technique in an acid-free, low-conductivity electrolyte. The modified GF exhibits a significantly higher activity towards O2 reduction. Up to 183.3% higher H2O2 yield is obtained by the anodized GF due to the increased concentrations of oxygen-containing groups and the hydrophilicity of the surface, which facilitates electron and mass transfer between GF and the electrolyte. Another significant finding is the ability to produce H2O2 at a high yield under neutral pH and low current intensity by the modified GF (35% of the charge need to produce the same amount by unmodified GF). Long-term stability testing of the modified GF showed a decay in the electrode’s activity for H2O2 production after 30 consecutive applications. However, the electrode regained its optimal activity for H2O2 production after a secondary modification by electrode polarity reversal. Finally, in situ electrochemically modified GF is more effective for removal of reactive blue 19 (RB19, 20 mg/L) and ibuprofen (IBP, 10 mg/L) by the electro-Fenton process. The modified GF removed 62.7% of RB19 compared to only 28.1% by the unmodified GF in batch reactors after 50 min. Similarly, 75.3% IBP is removed by the modified GF compared to 57.6% by the unmodified GF in a flow-through reactor after 100 min.

Improvement of the electro-bioremediation process of a non-polar herbicide-polluted soil by means of surfactant addition

Oxyfluorfen is a non-polar herbicide that may cause severe soil pollution. The present work studies the possible improvement due to surfactant addition in the efficiency of electro-bioremediation of a clay soil polluted which such a non-polar, low-mobility pollutant. Two-week-long batch electro-bioremediation experiments were performed in a bench-scale device. Oxyfluorfen-polluted soil (20 mg kg−1) was inoculated with an acclimated microbial culture, and several experiments were performed using different surfactant concentrations in the electrode wells (0.0, 2.5, 5.0, 10.0 and 20.0 g L−1 of SDS, sodium dodecyl sulphate). Experiments were performed under 1.0 V cm−1 and electrode polarity reversal. It was observed that the electro-osmotic flow (EOF) increased with SDS concentration and that SDS was successfully distributed across the soil, probably improving the oxyfluorfen mobility. Additionally, microbiological activity was fully maintained during the experiments. Electro-bioremediation without SDS removed 14% of the oxyfluorfen, while under 2.5 g L−1 SDS, the efficiency increased to 22% because of an expected improvement in the contact between the different species in the soil. However, higher SDS concentrations (between 10.0 and 20.0 g L−1) caused a decrease in the oxyfluorfen removal efficiency, as SDS is an easily biodegradable compound and was preferably used as substrate by the microbial culture instead of oxyfluorfen. Additionally, the use of high concentrations of SDS was clearly inefficient, as high amounts of the surfactant were lost through the EOF, and even low amounts of oxyfluorfen were removed to the electrode wells, which means that ex situ treatment of the polluted water would be needed.

Production of high purity water using membrane-free electrodeionization with improved resin layer structure

Abstract A novel membrane-free electrodeionization (MFEDI) process has been proposed and tested successfully for high purity water production in our previous work. However, to prevent the ions backward migration, the electrode polarity had to be reversed during regeneration, which complicated the regeneration process, reduced the regeneration efficiency and shortened the electrode life. In order to avoid the electrode polarity reversal and enhance the regeneration efficiency, the resin layer structure and the types of mixed resins were changed in this work to form a MFEDI without electrode polarity reversal which was then tested by desalinating the synthetic two-pass RO permeate of seawater with a total of 40 operational cycles. Results demonstrated that the backward migration of Na + ions was prevented effectively; the weak-type resins facilitated the water dissociation and promoted the regeneration of strong-type resins significantly. Both good purification and effective regeneration were achieved. The conductivity of effluent was 0.060–0.063 μS/cm only. The energy consumption and the water recovery were 0.24 kW h/m 3 water and 96.1%, respectively. Repetitive experimental results showed this MFEDI system could work stably.

Effect of NaCl concentration, pH value and tensile stress on the galvanic corrosion behavior of 5050 aluminum alloy

The effects of NaCl concentration, solution pH value and tensile stress on galvanic corrosion behavior of 5050 aluminum alloy coupled to 45 steel were investigated using a zero resistance ammeter. The influence and mechanism of various factors in the galvanic corrosion process were studied using electrochemical technology. The results show that there are large potential differences between 5050 aluminum alloy and 45 steel, and they have high galvanic corrosion tendency when making up a galvanic couple. The galvanic corrosion increases with increasing NaCl concentration, with decreasing pH of the solution and with increasing tensile stress applied on the material. In distilled water, the 5050 aluminum alloy potential is low at the beginning of test, resulting in the 5050 alloy acting as an anode and suffering corrosion. As the reaction progresses, the 5050 alloy's potential gradually increases and may exceed that of 45 steel. At this time, the electrode polarity reversal occurs and the 5050 aluminum alloy becomes the cathode. When the solution pH value is 2, the passive film loses its protective function for the matrix, resulting in serious corrosion. Tensile stress causes passive film breakdown on the 5050 aluminum alloy surface and increases the electrochemical potential, resulting in the increase of galvanic corrosion potential difference and serious galvanic corrosion.

Electrochemical transformation of trichloroethylene in aqueous solution by electrode polarity reversal

Electrode polarity reversal is evaluated for electrochemical transformation of trichloroethylene (TCE) in aqueous solution using flow-through reactors with mixed metal oxide electrodes and Pd catalyst. The study tests the hypothesis that optimizing electrode polarity reversal will generate H2O2 in Pd presence in the system. The effect of polarity reversal frequency, duration of the polarity reversal intervals, current intensity and TCE concentration on TCE removal rate and removal mechanism were evaluated. TCE removal efficiencies under 6 cycles h−1 were similar in the presence of Pd catalyst (50.3%) and without Pd catalyst (49.8%), indicating that Pd has limited impact on TCE degradation under these conditions. The overall removal efficacies after 60 min treatment under polarity reversal frequencies of 6, 10, 15, 30 and 90 cycles h−1 were 50.3%, 56.3%, 69.3%, 34.7% and 23.4%, respectively. Increasing the frequency of polarity reversal increases TCE removal as long as sufficient charge is produced during each cycle for the reaction at the electrode. Electrode polarity reversal shifts oxidation/reduction and reduction/oxidation sequences in the system. The optimized polarity reversal frequency (15 cycles h−1 at 60 mA) enables two reaction zones formation where reduction/oxidation occurs at each electrode surface.

Membrane-free electrodeionization without electrode polarity reversal for high purity water production

Membrane-free electrodeionization (MFEDI) has been tested successfully for high purity water production in our previous work. However, in order to prevent the constant backward migration of cationic or anionic ions, to achieve equally effective regeneration for both the cationic and anionic resins, and to get neutral concentrate, the electrodes had to be reversed during regeneration. This not only complicated the regeneration process, but also decreased the water recovery and the electrode lifetime. In this work, we improved the resin layer structure and investigated the regeneration performance of MFEDI without electrode polarity reversal. A total of 30 operational cycles were examined. Effective regeneration was achieved. The effluent conductivity was 0.055–0.060μS/cm only; the average pH value of the concentrate was 7.2; the water recovery was 90%; and the energy consumption was 0.71kWh/m3 water.

A laboratory feasibility study on a new electrokinetic nutrient injection pattern and bioremediation of phenanthrene in a clayey soil

Electrokinetic (EK) injection has recently been proposed to supply nutrients and electron acceptors in bioremediation of low permeable soils. However, effective pH control and uniform injection of inorganic ions have yet to be developed. The present study investigated a new EK injection pattern, which combined electrolyte circulation and electrode polarity reversal on a clayey soil. Soil pH could be controlled ranging from 7.0 to 7.6 by circulating the mixed electrolyte at a suitable rate (800mL/h in this study) without any buffer. Ammonium and nitrate ions were distributed more uniformly in soil by electrode polarity reversal. The developed electrokinetic injection technology was applied primarily in bioremediation of phenanthrene contaminated soil. Over 80% of the initial 200mg/kg phenanthrene in soil could be removed in 20d, and greater phenanthrene removal was achieved using electrode polarity reversal. Hence, the present study provides a promising electrokinetic injection technology for bioremediation of contaminated soils.

Reverse iontophoresis: A non-invasive technique for measuring blood lactate level

Blood lactate monitoring is beneficial to many patients, e.g. critical care patients. However, there are very few non-invasive or continuous monitoring systems for this parameter and significant clinical benefit could be achieved if such systems were readily available. Therefore, the aim of the current study was to investigate the possibility of non-invasively extracting lactate from blood through skin using reverse iontophoresis to monitor blood lactate levels in humans. In vitro reverse iontophoresis studies have indicated that the optimum switching mode for reverse iontophoresis of lactate is continuous direct current but application of current combined with electrode polarity reversal every 15 min was suggested to be used in humans. The reverse iontophoresis technique was applied to 10 healthy volunteers and lactate was successfully extracted through their skin into the methylcellulose gel of the electrodes. A moderate correlation ( r 2 = 0.6) between lactate concentrations in collection gels and lactate levels in the blood was observed after an outlier was removed from the regression equation. The result suggests that it may be possible to non-invasively monitor the blood lactate levels using reverse iontophoresis technique.

Application of an Enhanced Electrokinetic Ion Injection System to Bioremediation

Recently, electrokinetic injection has been applied in bioremediationto provide nutrients and TEAs in low permeable soil. However, an effective pH control system and a uniform injection system have yet to be developed. This study investigated an enhanced EK injection system, which is combined with electrolyte circulation and electrode polarity reversal on kaolinite. Soil pH was maintained continuously only by circulation of electrolytes in each chamber without any buffering solutions. Existing ions were distributed more uniformly in soil by electrodepolarity reversal. In view of microbiological degradation, the polarity reversal system resulted in better ion injection. Therefore, the novel electrokinetic nutrient and TEAs injectionsystem can be applied to in situ biodegradation more effectively than the conventional technique.